The invention relates to a means for the improvement of the moistening of offset printing plates.
Offset printing or surface printing plates have the property of being hydrophilic in the non-printing areas and hydrophobic (oleophilic) in the printing areas. In the course of the printing process, all areas are completely wetted with an aqueous moistening agent, then follows inking with the oily printing ink. The hydrophilic areas wetted by the moistening agent cannot be wetted by the hydrophobic, oily printing ink. In this manner an inked image forms on the plate, which is transferred via a rubber cylinder to the medium, i.e., the material which is to be provided with the printing.
The stability of this moistening agent film, i.e., a certain difference between the specific free surface energy of the printing plate material and the liquid, is essential to a clean distribution of the printing ink. Extensive theoretical descriptions of surface behavior, whose improvement forms the basis of the present invention, are to be found, for example, in Advances in Printing Science and Technology (Proceedings of the 17th International Conference of Printing Research Institutes, Saltsjobadan, Sweden, pp. 229-246, June 1983). Numerous attempts have been made to provide moistening agents with additives to improve wettability and hydrophilizing action. The state of the art is water-soluble synthetic and natural polymers, such as for example short-chain, even polyvalent alcohols, gum arabic, starch, alignates, dextrin, celluloses, and gelatines. In Internat. Bull. (1956, January), pp. 30-35, the use and action of these additives are described.
German published patent application No. OS 26 25 604 describes moistening agents on the basis of alcohol-and-water solutions containing univalent and polyvalent low-alkyl alcohols and glycol ethers, with molecular weights of 170 or less, in which not more than four successive carbon atoms are to be present. These moistening agents contain polyacrylamides, polyacrrylic acids and their salts, together in some cases with hydroxymethyl cellulose, in an amount of 0.001 to 5% by weight. Also added are metal nitrates and organic chelating agents.
According to German examined patent application No. AS 1 105 439, the moistening agent additives consist of silicon dioxide or mixed oxides, and polyvalent alcohols and citrate buffer are added if desired. No further data are given on the nature and manner of operation of the polyvalent alcohols.
In the presence of organic polymers, the polyalcohols have a tendency to form poor films, which interfere with the adsorption of the aqueous moistening agent onto the metal surface of the offset printing plates. Furthermore, these alcohols are problematical on account of their partial swelling action. Unless specific concentrations are precisely maintained, and if the atmospheric temperature and humidity fluctuate, water-soluble polymeric moistener additives of the state of the art often have a negative effect on the print quality as well as on the time the inks require for drying.
U.S. Pat. No. 4,711,670 issued on Dec. 8, 1987 to Walter R. Muller describes a moistening agent which contains a concentration of elastin and/or native collagen soluble in a slightly acid aqueous medium, of 0.01 to 3% of the total weight of the solution. This so-called elastin and native collagen has the property of having a low concentration of amine groups. This absence of amine groups would make the material anionic. Higher concentrations can lead to an undesirable formations of gel to the printing surface, and lower concentrations limits its effectiveness.
The optimum pH of the moistening agent is between 4.5 and 5.5. If the pH is lower, the printing plate can be chemically attacked, with the negative effects appearing in the printing ink and on the paper being printed. Also, the drying of the printing ink is retarded. At a pH above 7, saponification of the binding agent in the printing ink usually occurs.
Known buffers, such as sodium citrate-citric acid mixture or starch citrates, have proven suitable for the stabilization of the pH.
The moistening agent of the prior art patent can be applied to all conventional or alcohol- or spray-dampening rollers in the same way as known for usual agents, for example, gum arabic, carboxy-methyl cellulose or others.
One problem with the prior art moistening agent as described in U.S. Pat. No. 4,711,670, is that it is relatively difficult to extent its commercial possibilities.
An object of the present invention is to create a moistening agent for offset printing plates.
Another object of the present invention is to create a moistening agent for offset printing plates which has substantial commercial possibilities due to improved print quality and extended ink life.